The overall goal of this grant proposal is to identify which elements of hemodynamic load are the primary mechanical determinants of cardiocyte mass and to define the intracellular signaling mechanisms which couple hemodynamic lead to the hypertrophic response. Three hypothesis will be tested: 1) normal cardiocyte structure and mass are the direct and ongoing result of a normal cellular loading environment and not a fixed property of the cardiocyte; this normal loading environment can be recreated in an in vitro cell culture model, 2) the active force developed and bourne by the cardiocyte during contraction (after-load), the passive resting sarcomere length (pre-load), and the cardiocyte contraction frequency (tension-time index) are the primary mechanical determinants of cardiocyte mass, and 3) these determinants differentially regulate cardiocyte mass through integrin-dependent cellular signalling pathways couple alterations in hemodynamic load to regulation of cardiocyte structure and mass. These hypothesis will be tested via three specific aims: 1) define the role that integrin-mediated cardiocyte adhesion plays in the ability to maintain adult mammalian cardiocytes in long-term primary culture using our newly developed in vitro model in which cardiocytes are embedded in a gel matrix environment which closely mimics the in vivo composite structure of the myocardium, 2) define the primary dynamic mechanical determinants of cardiocyte mass, and 3) define the mechanisms by which these primary mechanical determinants of cardiocyte mass are coupled to the cellular signaling pathways which transduce mechanical input into the cardiocyte hypertrophic response.